Diabetes mellitus (DM) is a prevalent and devastating chronic disease, affecting >29 million people in the United States (US). Health insurance and continued access to healthcare services are essential for optimal DM care and management; therefore, it is hypothesized that Affordable Care Act (ACA) Medicaid expansions could substantially improve access to health insurance and healthcare services for patients at risk for DM or already diagnosed with DM (`with DM risk or DM'). The ACA called for every state to significantly expand Medicaid coverage by 2014; little is yet known about the impact of ACA Medicaid expansion on the prevention, treatment, expenditures, and outcomes of patients with DM risk or DM. In a 2012 legal challenge, the US Supreme Court ruled that states were not required to implement ACA Medicaid expansions; thus (by January 1, 2015), 28 states and the District of Columbia expanded Medicaid, while 22 states did not. This `natural experiment' presents a unique opportunity to learn whether and to what extent Medicaid expansion can affect healthcare access and services for patients with DM risk or DM. We will use this unprecedented natural experiment to expand our understanding of how Medicaid expansion impacts DM prevention, treatment, expenditures, and health outcomes. We will use electronic health record (EHR) data from the ADVANCE clinical data research network, which has data from 718 community health centers (CHCs), including 470 CHCs in 12 Medicaid expansion states and 248 CHCs in 9 non-expansion states. From this dataset, we will collect detailed information on changes in health insurance, service receipt, and health outcomes, with data spanning 9 years (pre- and post-expansion), comparing states that expanded Medicaid, and those that did not. Moreover, building on our prior work, we will link EHR data from 213 Oregon ADVANCE CHCs to Oregon Medicaid claims data to assess Medicaid expenditures among patients with DM risk or DM. The specific aims are as follows: Aim 1. Compare pre-post insurance status, overall visits, and chronic disease management visits among patients with DM risk or DM, in expansion versus non-expansion states. Aim 2. Compare pre- post receipt of primary and secondary DM preventive services (e.g., screening for obesity, lipid levels, glycosylated hemoglobin) among patients with DM risk or DM, in expansion versus non-expansion states. Aim 3. Compare pre-post changes in DM-related biomarkers (e.g., body mass index, blood pressure, lipid levels) in patients with DM risk or DM among newly insured (gained Medicaid in post-period), already insured (had Medicaid coverage in pre- and post-period), and continuously uninsured (pre- and post-period) patients in states that expanded Medicaid. Aim 4. Measure pre-post changes in Oregon Medicaid expenditures among newly insured compared to already insured patients with DM risk or DM. Findings will be relevant to policy and practice, informing further improvements in the US healthcare system to ensure access for vulnerable populations and mitigate disparities in DM prevention, treatment, and health outcomes.